Conventionally, as a panel structure of a vehicle body member in for example an automobile there has been employed a structure comprising an outer panel (called an external panel or outside sheet) and an inner panel (internal panel or inside sheet) which are combined to have a closed cross-sectional structure separated by a space.
Particularly, in a panel structure employed as for example the hood, roof or door of an automobile, the outer panel and inner panel, which is provided on the side of the outer panel facing the interior of the vehicle in order to reinforce this outer panel, are mechanically joined or are joined by welding or using for example an adhesive such as resin.
In a panel structure for a vehicle comprising such an inner panel and outer panel, in addition to or instead of the steel members that were conventionally employed, in order to achieve weight reduction, aluminum alloy sheet of high strength and high moldability such as 3000 grade, 5000 grade, 6000 grade or 7000 grade aluminum “aluminum” will hereinbelow be abbreviated as Al) alloy sheet as specified by AA or JIS has started to be used.
Recently, with a view to protection of pedestrians, there is a trend to mandate safety on head impact as a design requirement in relation to hoods. Various techniques have been proposed in relation to beam type hood structures (Laid-open Japanese Patent Application No. 7-165120, Laid-open Japanese Patent Application No. H. 7-285466, and Laid-open Japanese Patent Application No. H. 5-139338). Also, regarding impact-proof properties in relation to adults' heads and children's heads, the EEVC (European Enhanced Vehicle-Safety Committee) has laid down an HIC value of no more than 1000 as a requirement in respect of hoods (EEVC Working Group 17 Report, Improved test methods to evaluate pedestrian protection afforded by passenger cars, December 1998).
The present inventors have already applied for a patent (Laid-open Japanese Patent Application No. 2003-205866) in respect of a corrugated inner panels both in the case where the corrugated cross-section is regular and the case of an irregular spline type inner panel. Such a corrugated inner panel is a more preferred construction for pedestrian protection than a beam type inner panel or cone type inner panel in that, even though the clearance from the outer panel to rigid bodies such as the engine is smaller, it makes possible a further lowering of the HIC value. Specifically, with the invention according to the previous application using such a corrugated inner panel, the desired object was achieved of a vehicle body panel structure offering excellent protection for pedestrians. However, for further pedestrian protection, further reduction in the HIC value is desired.
On the other hand, many techniques have been disclosed relating to perforated-sheet sound absorbing panels, based on the Helmholtz resonance principle. The basic principles are indicated in acoustics textbooks. A resonant frequency is determined by simple expressions from for example the sheet thickness, hole diameter, aperture ratio, and thickness of the back air layer. A prescribed sound absorption performance can be obtained by determining these dimensions in accordance with the frequencies to be absorbed. The following provisional formula is indicated for a perforated-sheet sound absorption structure in Laid-open Japanese Patent Application No. H. 6-298014.f=c/2π×√(β/(t+1.6b)d)  [math 1]
Where, f is the frequency, c is the speed of sound, β is the aperture ratio, t is the sheet thickness, b is the aperture radius (radius of the holes) and d is the thickness of the back air layer.
It has been discovered that the sound absorption characteristic improves as the holes are made smaller and a high sound absorption performance is obtained due to viscous attenuation at hole diameters of 1 mm or less (H V Fuchs and X Zha: The application of micro-perforated sheets as sound absorbers with inherent damping, Acustica, 81, 107-116, 1995). As an example, in Patent document 1 (Laid-open Japanese Patent Application No. 2001-199287), it is stated in claim 4 that the hole diameter is 0.1 mm to 3 mm.
Although, in Patent document 2 (Laid-open Japanese Patent Application No. 2003-50586), Ueda, Tanaka and Uzuno et al specify that a suitable hole diameter is no more than 3 mm, with an aperture ratio of no more than 3%. This can easily be found assuming that the absorption frequency range is no more than 1000 Hz, as specified in Laid-open Japanese Patent Application No. 2001-122050. A perforated sound absorption structure is proposed therein wherein an external panel and internal panel are formed arranged facing each other, and the frequency bandwidth for which the noise absorption effect provides a noise absorption rate of at least 0.3 is set at 10% or more of the resonant frequency. In a range of panel thickness of the internal panel of 0.3 mm to 1 mm a range of aperture ratio of 1% to 5%, and a range of hole diameter of 0.5 mm to 3 mm, the effect on absorption rate of these parameters is investigated. In this case, it appears that, if the aperture ratio is no more than 3% and the hole diameter is no more than 3 mm, in particular if the hole diameter is no more than 1 mm, a fully satisfactory noise absorption effect is obtained i.e. the prescribed effect appears to be obtained. In the above publication, there is further proposed a perforated sound absorption body structure wherein at least two internal panels are provided, with an intervening air layer. However, a vehicle body hood structure that satisfies the requirements of both a sound absorption structure and a pedestrian protective structure has not as yet been developed.
In Patent document 3 (Laid-open Japanese Patent Application No. H. 6-298041), Patent document 4 (Laid-open Japanese Patent Application No. H. 6-81407) and Patent document 5 (Laid-open Japanese Patent Application No. 2000-276178), it is disclosed that wide-band sound absorption properties can be obtained by superimposing a plurality of flat sheets and perforated sheets of curved surface shape or perforated sheets of flat sheet shape, and providing a plurality of back air layers of varying cross-sectional shape. This is because, with such a structure, a wide range of resonant frequencies exist due to the changes in thickness of the back air layers, and, as a result, a sound absorption characteristic having a peak which appears in the case of only a single perforated sheet, is eliminated, and a substantially uniform wide-band sound absorption characteristic can be obtained over a wide frequency range.
Regarding a vehicle body hood structure that satisfies the requirements of both a sound absorption structure and a pedestrian protective structure, there are Patent document 6 (Laid-open Japanese Patent Application No. 2003-226264), Patent document 7 (Laid-open Japanese Patent Application No. 2003-252246) and Patent document 8 (Laid-open Japanese Patent Application No. 2003-261070).
In general, impact-resistance in respect of the head is evaluated using the HIC value described below (head performance criterion) (Automobile Technology Handbook Part 3, Test Evaluation Edition, Second Edition, 15 Jun., 1992, compiled by the Automobile Technology Association).HIC=[1/(t2−t1)∫t1.t2adt]2.5(t2−t1)  [math 2]
Where, a is the triaxial resultant acceleration (units: G) at the center of gravity of the head, t1 and t2 are time-points such that 0<t1<t2, and the action time (t2−1) is specified as no more than 15 msec at the time where the HIC value is a maximum.
In the EEVC Working Group 17 Report, as the condition to be satisfied in respect of the hood, an HIC value of no more than 1000 is respectively laid down for impact-resistance in respect of adults' heads and children's heads. The head impact speed that is therein specified in the case of a head impact test is 40 km/hour, an adult head being specified as of weight 4.8 kg, external diameter 165 mm, impact angle 65°, and a child's head being specified as weight 2.5 kg, external diameter 130 mm, impact angle 50°.
In the case of head impact, the head of a pedestrian collides first with the outer panel. Next, as deformation proceeds, reaction force is transmitted through the inner panel to rigid components such as the engine in the engine room, resulting in excessive impact force being generated at the head. The acceleration acting on the head comprises a first acceleration wave generated mainly by impact with the outer panel (generated during the 5 msec from the start of the impact) and a second acceleration wave (generated about 5 msec later than the start of the impact) generated on impact of the inner panel with a rigid object. The magnitude of the first acceleration wave is determined mainly by the elasticity and rigidity of the outer panel and the magnitude of the second acceleration wave is determined mainly by the elasticity and rigidity of the inner panel. The kinetic energy of the head is absorbed by deformation energy of this outer panel and inner panel. However, if the distance of movement of the head exceeds the clearance of the outer panel and the rigid object such as the engine, the head directly receives the reaction force from the rigid object and so receives an excessive impact force greatly exceeding the limiting value of 1000 of the HIC value, resulting in fatal damage.
Accordingly, it is necessary that it should be possible to decrease the HIC value even if the distance of movement of the head is small (problem 1). First of all, it is true that the larger the clearance between the outer panel and the rigid body such as the engine, the larger can be made the distance moved by the head, and this is beneficial in terms of reducing the HIC value. On the design of the hood, the clearance between the outer panel and the rigid body such as the engine has a limit itself. So a hood construction is sought that makes possible reduction in the HIC value even with a small clearance and small distance of head movement.
In particular in the case of head impact of an adult, the impact conditions are more severe than in the case of head impact of a child. So it is necessary to provide an excessively large clearance exceeding the allowed design range with regard to clearance from the outer panel to the surface of a rigid body, and this presents a problem (EEVC Working Group Report p 17).
Furthermore, it is considered to be a problem that it is very difficult to satisfy an HIC value of 1000 in respect of both children and adults, which have different impact characteristics on the WAD1500 line (line at the leading end of the vehicle from the road surface to the hood impact position with a distance of 1500 mm from the contour line) on which impact with either a child's head or an adult's head is possible. In particular, in the case of the hoods of large sedans, the WAD1500 line is directly above the engine where the clearance between the outer panel and rigid body surface is small, so effective counter-measures are desired regarding improvement of impact-resistance (EEVC Working Group 17 Report).
Next, it is necessary that the HIC value should be uniform irrespective of impact location (problem 2). Regarding the position of head impact, the HIC value is large at a position directly above the frame in the case of a beam type hood construction and at the position of the cone vertex section in the case of a cone type hood construction. This is because these locations have high local rigidity and little deformation when colliding with the rigid body portion and receive high reaction force from the rigid body. Consequently, from the point of view safety, a hood construction is desired in which a roughly uniform HIC value is obtained irrespective of the location of impact.
In addition, it is necessary that it should be possible to employ Al alloy material, which makes it possible to reduce vehicle weight (Problem 3). Excellent head impact-resistance is necessary even if Al alloy material is employed as the hood material for purposes of weight reduction. Al alloy material is frequently employed for purposes of hood weight reduction, but, in this case, compared with the use of iron-based material, is considered to be generally disadvantageous from the point of view of pedestrian protection. This is caused by the fact that the elasticity coefficient and specific gravity of Al alloy material are both about one-third of those of steel material, so the sheet rigidity and the weight of a hood made of Al alloy as a panel structure are insufficient, compared with a hood made of steel, for the kinetic energy of the head to be absorbed by the hood.
The bending rigidity of a sheet member is proportional to ET3 (where E is the Young's modulus and T is the sheet thickness) and the sheet rigidity is proportional to ET. When an iron-based material (Young's modulus Es, sheet thickness Ts and specific gravity γs) is substituted by an Al alloy material (Young's modulus Ea, sheet thickness Ta, specific gravity γa) usually, the sheet thickness is determined such that the bending rigidity is the same. In this case, EaTa3=EsTs3, Ea/Es=⅓, and Ta/Ts=31/3=1.44. The sheet rigidity ratio of a hood made of aluminum alloy and a hood made of steel is (EaTa)/EsTs=1.44/3=0.48, and the specific gravity ratio is likewise (Taγa)/(Tsγs)=1.44/3=0.48, so the sheet rigidity and weight of an aluminum hood are only 0.48 times those of a steel hood. As a result, in the problem of impact of the head and hood, the distance moved by the head is increased, making impact with a rigid body more likely, and decreasing the energy absorption of the outer panel in the first acceleration wave, while increasing the second acceleration wave; as a result, the HIC value, with a conventional hood construction, is increased, making it extremely difficult to satisfy the limiting value in respect of the HIC value.
Of course, if Ta is made three times Ts, the sheet rigidity ratio and weight ratio will both be equivalent to those of a steel hoods but costs will be excessively increased so that such a design cannot be adopted.
Thus it is fairly difficult to satisfy the limiting conditions under head impact with these conditions if aluminum alloy material is employed for the hood. Of course, if a hood construction could be found whereby these conditions were satisfied with aluminum material, such a construction would make possible even further reduction of the HIC value in the case of a steel hood adopting this construction.
Patent document 1: Laid-open Japanese Patent Application No. 2001-193287
Patent document 2: Laid-open Japanese Patent Application No. 2003-50586
Patent document 3: Laid-open Japanese Patent Application No. H. 6-298014
Patent document 4: Laid-open Japanese Patent Application No. H. 6-81407
Patent document 5. Laid-open Japanese Patent Application No. 2000-276178
Patent document 6: Laid-open Japanese Patent Application No. 2003-225264
Patent document 7: Laid-open Japanese Patent Application No. 2003-252246
Patent document 8: Laid-open Japanese Patent Application No. 2003-261070